Method for measuring position and position measuring device for carrrying out said method

ABSTRACT

A position measuring system that includes a scale and a scanning device that scans the scale. A light source, which emits a light pulse upon receipt of a request signal and an optical fiber that transmits the light pulse from the light source to the scanning device and for illuminating the scale. At least one photo detector that detects the light pulse affected by the scale as a function of its position.

[0001] Incremental or absolute position measuring systems are used fordetecting the definite position of moved objects on machines, such asmachine tools or wafer steppers, for example. In connection with this,the position measuring system must measure the position of the object atfixed definite times and inform the electronic control device whichcontrols the movement sequence. The times are mostly defined by theelectronic control device with the aid of trigger pulses. These triggerpulses are provided to the position measuring system, or to itselectronic evaluation device, which then stores an internal count andtriggers the A/D converters for signal interpolation by storing, ortaking over, instantaneous values of the scanning signals with the sameperiod, which are phase-shifted in respect to each other, and which areanalog-digitally converted. At the end, the internal signal processingdevice in the electronic evaluation device outputs a measured positionvalue which was not present exactly at the time of the triggering, butinstead at a time which was displaced by the amount of storage time.Typical storage times are a few μs.

[0002] The continuously increasing displacement speeds, and theincreased demands made on accuracy at the same time require,particularly at high speeds, increasingly shorter storing times, andabove all an extremely small fluctuation of the storing time (storingjitter). The latter will be explained by means of the example of a waferstepper, which was taken from an article by P. Kwan, U. Mickan, M.Hercher “Nanometergenaue Positionsmessung in allen Freiheitsgraden”, F&M108 (2000) 9, pp. 60 to 64. At a displacement speed of 2 m/s and astoring jitter of only 1 ns, the position uncertainty caused by this isalready 2 nm, which represents a considerable loss of accuracy inconnection with such applications. On the other hand, a storing jitterof less than 1 ns makes extremely high demands on the electronicevaluation device and the position measuring system. The followingeffects must be taken into account in connection with position measuringsystems:

[0003] All analog amplifiers required for signal processing upstream ofthe A/D converter have limited bandwidths, and therefore delay thescanning signals to a considerable extent. Small amounts of drift of thecomponents used because of the effects of temperature or aging affectthe signal running times and therefore greatly contribute to storingjitter. Moreover, the signal running times are a function of the inputfrequency, and therefore the displacement speed, which can produceadditional contributions to the storing jitter.

[0004] The A/D converters also contribute to the storing jitter, becausethey do not measure the applied voltages exactly in relation to theswitching flanks of the carrier pulses.

[0005] As a rule, the scanning signals are phase-shifted by 90° inrelation to each other. The sine signal, as well as the cosine signal,must have the same storing time, otherwise an effective storing time ofthe position measuring system which differs from the exact position isobtained, which fluctuates between the storing time of the sine signaland the cosine signal. Regarding the position determination within asignal period, the respective scanning signal located in the vicinity ofits crossover is decisive, since it shows the greatest change inposition, or phase relation, in this range.

[0006] A position measuring system is described in DE 44 10 955 A1, inwhich the light source is supplied with a strong current at the time anexternal trigger signal is present as a request signal. The disclosedsynchronization of the light source with external trigger pulses issuited only for low demands made on the storing jitter, because thesupply of the trigger pulses (request signal) to the light sourcelocated in the position measuring system takes place there by means of awire connection from an external electronic tracking device (electroniccontrol device). With customary cable lengths of 0.5 to 20 m, this doesnot assure a sufficient running time stability in the 10 ns range andbelow, and can therefore not be used in demanding applications. In thisconnection it should be noted that low-jitter trigger pulses are mostlyavailable only directly at the electronic components which generatethem.

[0007] It is therefore the object of the invention to disclose a methodfor position measurement and a position measuring system for executingthe method, which assure a precise position measurement.

[0008] This object is attained by means of a method with thecharacteristics of claim 1, or with a position measuring system with thecharacteristics of claim 6.

[0009] Advantageous embodiments of the invention are disclosed in thedependent claims.

[0010] Extremely small storing jitters can be achieved by means of themeasures in accordance with the invention.

[0011] The invention will be explained in greater detail by means ofexemplary embodiments.

[0012] Shown are in:

[0013]FIG. 1, a first position measuring system in accordance with theinvention, and

[0014]FIG. 2, a second position measuring system in accordance with theinvention.

[0015] The position measuring system 100 in FIG. 1 consists of a scale 1with a graduation 2, which can be opto-electrically scanned and movedrelative to a scanning device 5 in the measuring direction X. Thegraduation 2 can be embodied so that it can be scanned by transmittedlight or by incident light. The scale 1 can furthermore be designed forincremental or absolute linear or angular measuring. The light requiredfor illuminating the scale 1 is conducted through an optical fiber 3 tothe position measuring system 100. The light source 4 of the opticalposition measuring system of the invention is not installed directly inthe scanning device 5 and instead is located at the place where alow—jitter trigger pulse I—also called a request signal—is available,i.e. preferably in the vicinity of, or integrated into an electroniccontrol device 200, which generates the trigger pulse I. The triggerpulse I is now transmitted via a digital, and therefore fast driverstage 6 to the light source 4, which therefore synchronously transmits alight pulse IL of a pulse length of 25 psec to 5 nsec. In the electroniccontrol device 200, this light pulse IL is coupled into the opticalfiber 3 and transmitted to the scanning device 5. In this case therunning time of the light pulse IL in the optical fiber 3 is veryconstant, in particular if single-mode fibers are employed. However,very good results are also achieved with multi-mode graded index fibers.In the scanning device 5, the light pulses IL are directed by means of asuitable optical scanning device onto the scale 1, and finally tophotodiodes 7.1, 7.2, 7.3. In this case it is irrelevant which opticalscanning device is used. In particular, imaging, as well asinterferential scanning methods are available. Photo-charges are nowgenerated in the photodiodes 7.1, 7.2, 7.3 by the short light pulses IL,which are transmitted to downstream-connected charge amplifiers 8.1,8.2, 8.3. The charge amplifiers 8.1, 8.2, 8.3 can be integrated eitherinto the downstream-connected electronic evaluation device 9, ordirectly into the scanning device 5. It is advantageous if they areintegrated into the scanning device 5, so that charge-amplified scanningsignals are conducted through lines 21, 22, 23. At their outlets theyprovide signal voltages, which are converted into digital signals byanalog-digital converters (A/DCs). The subsequent digital processingtakes place in the same way as in customary position measuring systemsin that within a period interpolation signals are determined fromseveral scanning signals of the same period, which are phase-shifted by120° or 90° in respect to each other. These processing methods are knownand will not be further explained here. Immediately following themeasurement, the charge capacitors of the charge amplifiers 8.1, 8.2,8.3 are reset (discharged), so that the next measurement can beperformed by sending a further light pulse IL to the scale 1. The shortlight pulse IL defines in an extremely exact manner the time of theposition determination. The downstream—located electroniccomponents—such as photodiodes 7.1, 7.2, 7.3, amplifiers, A/DCs 10,connecting cables, etc.—do not affect the result and can therefore bedesigned to be relatively slow, and therefore cost-effective.

[0016] Depending on the scanning method, laser diodes, VCSELs, LEDs,solid state lasers, superluminescence diodes, can be considered as lightsources 4. In this case a further advantage of the invention comes tothe fore, particularly in connection with semiconductor lasers: becauseof the pulsing, the laser becomes longitudinally monomodal and cantherefore no longer have interfering mode jumps. In connection withinterferential position measuring systems 100, such mode jumps causesudden and very interfering jumps in the indicated position already withslightly different optical path lengths of the interfering lightbeams.

[0017] For incremental scanning methods it is furthermore necessary todetermine the number of the crossed signal periods. In contrast toposition measuring systems in which the scale 1 is continuouslyilluminated, the realization of counters is here no longer directlypossible, since no scanning signals are available between two triggerpulses I. It is therefore suggested to trigger the position measuringsystem 100 very frequently, adapted to signal period and the maximumspeed, or acceleration, for example with 1 MHz. By means of this hightrigger rate it is possible to calculate speeds which can hardly changefrom one trigger time to the next from the interpolated fine positions.At a slow speed a sufficient number of measured values is obtained inone signal period so that it is possible to unequivocally detect a jumpto the next signal period, and a software period counter can thereforecount up. If the speed is increased, successive measured values can beseparated from each other up to a few signal periods, and a dependablecount of the signal periods with the aid of the software period counteris possible in spite of this. To this end, the speed is approximatelycalculated from previous fine positions, and the occurring signalperiods between two trigger times are determined from this. A hightrigger rate is advantageous. If the trigger rate of the electroniccontrol device 200 is too low, the electronic evaluation device 9 mustintersperse additional trigger pulses I.

[0018] The exemplary embodiment in accordance with FIG. 2 differs fromthe first one by the use of multi-mode optical fibers 11, 12, 13, forthe return of the light from the scanning light beams, which weremodulated at the scale 1, to the electronic evaluation device 9. In thiscase the photodiodes 7.1, 7.2, 7.3 are contained in the electronicevaluation device 9 for receiving the light transmitted back by theoptical fibers 11, 12, 13. In this way the scanning device 5 becomespassive, i.e. it is no longer connected by electrical cables and cantherefore also be employed in critical environments (high tension,discharges, explosive gases) without interference. Thereby it isfurthermore also possible to transmit signals of relatively highfrequency in a simple way.

[0019] The light pulse IL from the light source 4 can be used for thesimultaneous illumination of several scales, for example on severalshafts of a machine. In this case the optical fibers in which the commonlight pulse IL is supplied to the individual scales should have at leastapproximately the same lengths.

[0020] The trigger pulse I is an electrical or optical pulse, by meansof which sensors, and if required actuators, of a machine aresimultaneously synchronized in an advantageous manner. Positionmeasuring systems 100 with several shafts, distance sensors,acceleration sensors and speed sensors, i.e. sensors which are employedfor control, are counted among the sensors. The trigger pulse I issynchronized with the control cycle of the control unit of the machine,for example a numerically controlled machine tool.

[0021] If the trigger pulse I already is an optical pulse, the elementsidentified by the reference numerals 4 and 6 in FIGS. 1 and 2 aresuperfluous, an optical processing unit in the form of an optical fiberamplifier, optical switch or optical mixer can be provided in theirplace.

[0022] The highly accurate position measurement in accordance with theinvention can be combined with a second position measurement. Forexample, constant light of a wavelength differing from the light pulseIL is transmitted through the optical fiber 3, and the graduation 2and/or another graduation, or coding, is illuminated and scanned by thisconstant light. A rough position is then determined by scanning with theconstant light, and this position is refined by means of the light pulseIL. In this case the rough position can be determined by means of ahardware counter, and the position determination by means of the lightpulse IL takes place by means of a software counter, which provides theinstantaneous position on the basis of interpolation values of thescanning signals, which are phase-shifted in respect to each other.

1. A method for position determination, having the following methodsteps: generating a light pulse (IL) upon receipt of a request signal(I), transmitting the light pulse (IL) through an optical fiber (3) to ascale (1), and illuminating the scale (1) with the light pulse (IL),affecting the light pulse (IL) in a position-dependent manner by thescale (1), detection of the light pulse (1) affected by the scale (1) byat least one photo detector (7.1, 7.2, 7.3).
 2. The method in accordancewith claim 1, characterized in that the light pulse (IL) is generatedupon receipt of an electrical request signal (I).
 3. The method inaccordance with claim 1 or claim 2, characterized in that photo-chargesare generated in the photo detector (7.1, 7.2, 7.3) by the light pulses(IL), which are transmitted to downstream-connected charge amplifiers(8.1, 8.2, 8.3).
 4. The method in accordance with claim 3, characterizedin that the charge amplifier (8.1, 8.2, 8.3) is reset between theappearance of two light pulses (IL).
 5. The method in accordance withone of the preceding claims, characterized in that several photodetectors (7.1, 7.2, 7.3) are provided, each of which receives a partiallight beam of the light pulse (IL) affected by the scale (1).
 6. Aposition measuring system, having a scale (1) and a scanning device (5)for scanning the scale (1) a light source (4), which emits a light pulse(IL) upon receipt of a request signal (I), an optical fiber (3) fortransmitting the light pulse (IL) from the light source (4) to thescanning device (5) and for illuminating the scale (1), at least onephoto detector (7.1, 7.2, 7.3) for detecting the light pulse (IL)affected by the scale (1) as a function of its position.
 7. The positionmeasuring system in accordance with claim 6, characterized in that acharge amplifier (8.1, 8.2, 8.3) is connected downstream of at least onephoto detector (7.1, 7.2, 7.3).
 8. The position measuring system inaccordance with claim 7, characterized in that the at least one photodetector (7.1, 7.2, 7.3) is arranged together with charge amplifier(8.1, 8.2, 8.3) in the scanning device (5).
 9. The position measuringsystem in accordance with claim 6 or claim 7, characterized in that theat least one photo detector (7.1, 7.2, 7.3) is arranged in an evaluationdevice (200), and that the light pulse (IL), which is affected in aposition-dependent manner by the scale (1), is conducted via an opticalfiber (11, 12, 13) to the photo detector (7.1, 7.2, 7.3).
 10. Theposition measuring system in accordance with one of claims 6 to 9,characterized in that the optical fiber (3) is a single-mode fiber. 11.The position measuring system in accordance with one of claims 6 to 9,characterized in that the optical fiber (3) is a multi-mode graded indexfiber.
 12. The position measuring system in accordance with one ofclaims 6 to 11, characterized in that the light source (4) is asemiconductor laser.
 13. The position measuring system in accordancewith one of claims 6 to 12, characterized in that several photodetectors (7.1, 7.2, 7.3) are provided, each of which receives a partiallight beam of the light pulse (IL) affected by the scale (1).